This invention relates to a digital signal processing apparatus and, more particularly, to a digital signal processing apparatus having a trigger means for extracting a specific signal from signals with amplitude, pulse width, period or the like varying along the time base in a complex manner.
A conventional digital oscilloscope as a digital wave observation apparatus adapting digital signal processing techniques is illustrated in FIG. 1. Reference numeral 101 denotes a trigger generator. In the generator 101, a trigger level preset by a volume control 102 is compared by an analog comparator 103 with an input signal. When the input signal is larger than the trigger level preset by the control 102, the trigger signal is generated by the generator 101. The trigger signal is supplied to a signal fetching controller 104. The controller 104 generates a data fetch control signal to a wave memory 106 in response to the trigger signal. Wave data of the input signal converted by an A/D converter 105 is stored in the wave memory 106. The wave data of the input signal stored in the memory 106 is read out under the control of a display controller 107 and is displayed on a display 108.
FIG. 2 shows a waveform for explaining how a trigger signal is generated in the oscilloscope of FIG. 1. In this case, a waveform portion I is triggered with level L.
For this reason, a waveform portion II surrounded by the broken rectangle of FIG. 2 is triggered with another signal wave. In other words, the desired portion II cannot be triggered at a desired moment. As a result, the desired wave cannot be observed, resulting in inconvenience.
This is because the input signal is stored in the wave memory 106 at the predetermined trigger level L although the amplitude, width and period of the input signals vary along the time base in a complex manner.